At first we thought it was a child, perhaps three years old. But a closer look showed that the tiny, fragile bones we had just laid bare in a spacious cave on the Indonesian island of Flores belonged to a full-grown adult just over three feet (one meter) tall.

Had we simply found a modern human stunted by disease or malnutrition? No. The bones looked primitive, and other remains from Liang Bua, which means "cool cave" in the local Manggarai language, showed that this skeleton wasn't unique. It was typical of a whole population of tiny beings who once lived on this remote island. We had discovered a new kind of human.

Back in the lab, where we analyzed the bones and other artifacts, the full dimensions of what we had discovered began to emerge. This tiny human relative, whom we nicknamed Hobbit, lived just 18,000 years ago, a time when modern humans—people like us—were on the march around the globe. Yet it looked more like a diminutive version of human ancestors a hundred times older, from the other end of Asia.

We had stumbled on a lost world: pygmy survivors from an earlier era, hanging on far from the main currents of human prehistory. Who were they? And what does this lost relative tell us about our evolutionary past?